The common Shimano Dynamo hubs don't actually use sealed cartridge bearings, see the DH-3N71 and DH-3D72 techdocs. Shimano dynohubs do use good seals, however, and should hopefully be maintenance-free for many thousands of miles.

Alistair Spence has a good exploded view of an Alfine DH-S500, which is very similar to the DH-3D71. He also links there to a disassembly guide PDF, the original of which can be seen (minus images) in the internet archive. The steps are basically the same as a standard front hub, with the caveat that it is important to be careful to avoid breaking the axle wire.